Migratory Birds and Sleep: How They Manage Long Journeys
Once breeding is complete, thousands of birds around the world, from tiny warblers and seabirds to cranes, swans and pelicans, undertake migration journeys to wintering grounds.
Ranging from a few kilometers downslope to the epic 30,000-km (48,000-mi) flight of the Arctic Tern, birds cross vast tracts of land and open ocean to reach their destinations. Read on to find out more about the exhausting feat of migration and how birds manage their sleep en-route.
Bird migration is triggered in some species by a change in climate conditions, an availability of food, or a need for shelter in a different habitat. In the northern hemisphere, non-resident birds will typically arrive on their breeding grounds in the spring, breed and raise their young, and then return to wintering grounds as the cooler fall weather approaches.
Undertaking these marathon journeys is physically demanding, with much preparation needed to ensure they build up enough reserves to survive the experience.
However, regardless of a bird’s physical condition when setting out on a migration journey, it’s undeniable that the journey will take its toll on a bird’s health, and getting sufficient sleep to restore energy levels is crucial.
Some birds are on a mission to reach their destinations as quickly as possible and do not wish to delay their arrival, covering significant distances without a break. But on little or limited sleep, these efforts can become even more of a challenge.
A flock of Snow Geese during migration. Bird migration is triggered in some species by a change in climate conditions, an availability of food, or a need for shelter in a different habitat
The Phenomenon of Migration
Although not all birds migrate, many species have different requirements for breeding habitats and wintering territories. Find out more about why birds migrate and the logistics of these seasonal journeys below.
Migrations are primarily dictated by the availability of food, shelter and warmth. Some bird species have specialized requirements for breeding grounds, which may not support their needs all year round.
Northern landscapes offer ideal nesting territories for a range of birds, including hawks, gulls and buntings. But in winter, these environments become inhospitable and food resources become in short supply, leading these species to head south in search of warmer conditions and more abundant food.
Undertaking migration is physically demanding, and birds will spend a period of time before embarking on their journeys preparing their bodies and stocking up on the fatty acids needed to power their flight muscles.
Finding food en-route is never guaranteed, birds therefore need to conserve whatever energy stores they have established. Similarly, in normal conditions, overnight roosting periods bring a chance for birds to rest and recover, although again, while migrating, opportunities for quality sleep may also be few and far between.
A Red-tailed Hawk in-flight. Migrations are primarily dictated by availability of food, shelter and warmth
Balancing Flight and Rest
The Guinness World Record for the longest continuous flight was set at 8,345 miles by a Bar-tailed Godwit in January 2023. The non-stop migration flight, from Alaska to Tasmania, Australia took 11 days, without a break for food or sleep.
Rest is directly linked to avian health, and the physically exhausting demands of any migration flight will have an impact on a bird’s fitness and general health.
Some birds may be in a position to take brief rest breaks during migrations, landing for a period of time at stopovers or staging points, where they are able to catch up with some of their missed sleep, restore their energy levels and ensure they are in decent condition to continue with their journeys.
Migrating birds use a technique that allows them to briefly shut off one side of their brain and benefit from partial rest while continuing to fly. During these microsleeps, flight and breathing continue uninterrupted, and they are able to maintain a degree of awareness of their environment.
In rare examples, birds may be able to switch off both sides of their brain simultaneously, for example, the frigatebird. Frigatebirds are able to fly without stopping for a break for up to two months but exist on around one hour of sleep per day, significantly less than if they were on land.
A Bar-tailed Godwit. The Guinness World Record for the longest continuous flight was set at 8,345 miles by a Bar-tailed Godwit in January 2023
Sleeping While Soaring
Unlikely as it sounds, many birds continue their migration flights in a state of semi-sleep, where they are not fully conscious of their actions or movements but aware enough to maintain a degree of vigilance and alertness to their surroundings.
Sleeping while soaring allows them to benefit from the recovery periods associated with full rest while reaching their destinations sooner by not stopping for a break.
A phenomenon known as Unihemispheric Slow-Wave Sleep (USWS) allows birds to switch one-half of their brains into a state of slow-wave sleep, while the other half remains in a state of wakefulness. The sleeping “hemisphere” enters slow-wave sleep, characterized by low-frequency, high-amplitude brain waves.
The restorative properties of sleep begin, even though one-half of the brain is still active, which is an important survival mechanism in situations where round-the-clock vigilance is needed.
While in a state of half-brain sleep, birds remain able to navigate their way along migration routes, relying largely on the imprinted memory of familiar route markers. They are also vigilant enough to be aware of any changes to their immediate environment that may threaten their survival, including obstacles and predators.
An Alpine Swift. Sleeping while soaring allows birds to benefit from the recovery periods associated with full rest while reaching their destinations sooner by not stopping for a break
The Role of Circadian Rhythms
Circadian rhythms are important to the survival of all birds, not solely those that undertake migrations. These biological processes are responsible for regulating a bird’s behavior and lifestyle and ensuring their activities are in sync with the natural day-night cycle. Read on to discover more.
Circadian rhythms act as internal timekeepers for birds, helping them to regulate hormone production, their sleep-wake cycles and their metabolisms, in synchronization with the natural 24-hour day-night cycle. This ensures that they are physiologically prepared for the demands of long-distance flights.
During migration, a bird’s biological clock plays an important role in the timing of their flights and accurate navigation over long distances. Circadian rhythms help them to anticipate changes in environmental conditions that may affect their migration route, including weather patterns and the availability of food en route.
Circadian rhythms also enable birds to regulate their energy levels and rest periods during migration. Nocturnal flights are common among many songbird species, which allows them to take advantage of cooler temperatures and reduced risks of predation. During the day, birds may break their journeys to rest and refuel, conserving energy for the next stage of their migration.
A flock of Sparrows taking-off. Circadian rhythms enable birds to regulate their energy levels and rest periods during migration
Impact of Sleep Deprivation
Covering even short distances at the end of a busy breeding season can be physically demanding for birds, with increased amounts of energy required for longer sustained flights. With limited sleep, the pressure to focus on navigation and general survival is naturally enormous. Learn more about how lack of sleep may impact a bird’s ability to migrate safely and successfully below.
Sleep deprivation disrupts the normal restorative functions needed to repair and reset a bird’s health and fitness. Lack of sleep can weaken a bird’s immune system, impair its metabolism, and increase its stress levels. These factors can make birds more susceptible to diseases, which can potentially impact their ability to successfully complete their migration flights.
Sleep plays a vital role in the navigation abilities of migratory birds, as it can disrupt their ability to process and interpret the natural cues and landmarks used to plot their journeys effectively. Functioning with insufficient sleep can impact memory and spatial awareness, both essential for navigation, causing sleep-deprived birds to experience difficulties in safely reaching their destinations.
The drop in alertness caused by sleep deprivation can lead to birds having slower reaction times and, therefore an increased likelihood of accidents. The risk of collisions with buildings, power cables and vehicles along their migration routes increases in birds functioning on less sleep than they need.
This also can have serious longer-term implications on a bird’s ability to breed. Birds need to arrive on breeding grounds in the best physical condition possible to attract a mate, engage in courtship and pass on strong genes to any offspring.
Birds that arrive in suboptimal condition, already exhausted, stand a lower chance of finding a mate and breeding successfully. Rest stops on a migration journey are therefore not only valuable but in many birds vital in their safe arrivals on their wintering or breeding grounds.
USWS allows birds to take longer migration flights, including over water, where suitable rest stops are not always available, and although birds using this process are never fully rested, it does give them a temporary boost that allows them to make progress on their journeys without having to stop and rest physically.
A pair of Cape Gannets at their breeding grounds. Birds need to arrive on breeding grounds in the best physical condition possible to attract a mate, engage in courtship and pass on strong genes to any offspring
Navigational Aids and Rest
You may have wondered how migrating birds seem to always know in which direction they should be traveling. For some species, routes are learned and imprinted from accompanying their parents on their first flight to wintering grounds. For other species, migration is inherent and supported by following natural cues and visual landmarks on their journey.
The position of stars in the night sky and the Earth’s magnetic fields are important navigational aids to migrating birds, which rely on these markers to signpost their routes to and from wintering grounds. Routes are mapped out in their minds by following these cues and watching out for familiar landmarks. These routes are imprinted in their memories, and with an alert, functioning mind, staying on course is never normally an issue.
Birds also use these virtual signposts and landmarks to identify suitable stopping points on their routes, registering locations on their journey where they benefited from a break during a previous migration. Rest breaks are long overdue when staying on course becomes a challenge.
Brief periods of recovery are vital when birds start to become less aware and are no longer alert enough to notice the key navigational cues that guide them to safely touch down.
A flock of Sandhill Cranes in-flight. Birds use virtual signposts and landmarks to identify suitable stopping points on their routes, registering locations on their journey where they benefited from a break during a previous migration
Rest Stops and Their Significance
Not all migrations are possible to complete nonstop, and rest breaks and stopovers provide an important chance for birds to refuel and recharge. Learn more about how birds choose and take advantage of rest stops during migration.
Rest breaks during migration flights allow birds to briefly rest, recover, top up their energy reserves and avoid any bad weather conditions that may make continuing to fly particularly difficult. Stopover sites are chosen that offer suitable food resources and shelter, and often their ability to support vast numbers of migrating birds.
Some rest stops become staging points for tens of thousands of birds each spring and fall, including the Wadden Sea, along the coast of Germany, Denmark and the Netherlands, where up to 12 million wetland birds pause for respite each year before carrying on with their onward migrations.
Many birds migrate in stages rather than tackling the entire journey without a break. Stopover sites serve as ‘service stations’ on migration routes and offer the necessary resources and natural habitats for anything between a couple of days to several weeks while birds stock up on energy reserves for the next leg of their journey.
Birds have been migrating along the same routes between breeding territories and wintering grounds for many generations and if resources at these stopovers en-route are lost, it can mean that birds will become unable to successfully continue with the next step journey, becoming stranded in unsuitable habitats.
Juvenile geese and swans accompany their parents on their first migrations and learn the best places to break their journeys en-route. In other bird species, for example, dunlins and curlews, migration routes are not taught but followed instinctively.
A flock of Barnacle Geese flying in the Wadden Sea National Park in Denmark
Human-made Challenges
As well as the natural challenges faced by migrating birds, a number of problems introduced by humans can also affect how successful their annual treks are.
Many birds migrate by night and find their way using visual landmarks, stars in the night sky and cues from geographical features. Human neighbors may unwittingly introduce some potentially highly challenging obstacles to their path. To find out how, please read on.
Birds rely on a number of visual signals and markers to navigate their migration flights and ensure they reach their chosen destination. Many species are nocturnal migrants, with their annual journeys between wintering grounds and breeding territories triggered by certain natural conditions.
The existence of additional light sources illuminating the skies can disrupt these journeys with deadly consequences. Birds may mistime their migrations if they are confused by any extra and unnatural light, meaning that when they arrive at their breeding or wintering grounds, the resources they need aren’t available.
Traffic, power cables, and tall buildings all play a role in creating a confusing, disorienting skyline that may disrupt the preferred natural flight paths of migrating birds. Both on land and in the sky, the presence of moving vehicles can potentially throw individual birds and flocks off course, with unexpected noise, lights and sudden movements that may make it impossible to avoid being struck.
Power cables also pose a collision hazard, with entanglement and electrocution a serious risk. Records show that in Germany alone up to a million migratory birds each year are involved in fatalities caused by power cables.
A flock of Canada Geese migrating. Many birds migrate by night, and find their way using visual landmarks, stars in the night sky and cues from geographical features
Summary
One of the most impressive feats of the animal kingdom is the long-distance migration routes undertaken each year by birds, covering thousands of miles, dozens of countries and large expanses of oceans.
But even short-distance migrations are a source of wonder, with birds enduring exhausting conditions and completing complex routes using memory and instinct.
Understanding how birds develop and follow migratory routes is the key to helping protect these corridors, ensuring that no additional stresses or challenges are placed in the way during their long-distance journeys.
The UN’s 2022 scheme to raise awareness of the major threats that birds face during migration launched with the tagline “Dim the lights for birds at night” hoping to promote the impact of light pollution on migrating birds.